This invention relates to an electrochemical cell, a current collector for the cell and a method for cell construction. In particular, the invention relates to a hermetically sealed cell having a reactive cathode and a lithium anode. The electrochemical cell may be used to power body implantable medical devices such as heart pacemakers.
In constructing an electrochemical cell for use in implantable medical devices, a known method of making a cathode pellet is to compress a mixture of powdered metal oxide, a conductive matrix such as graphite or carbon black and a binding material such as polytetrafluroethylene (PTFE). In such a cell, it is essential for uniform discharge of the cell to maintain good contact between the cathode current collector and the cathode material. One known way of providing such contact is to imbed the current collector inside the cathode powder mixture and then compress the mixture into a pellet.
A drawback of imbedding the current collector in the cathode material is that the volume of the cathode typically expands as the cathode is discharged. This expansion of the cathode can cause degradation of the contact between the cathode and the current collector, causing changes in overall cell impedance as the cell is discharged. During latter stages of discharge, contact degradation may cause such an increase in impedance that it causes a significant decrease in cell capacity. Since high reliability in a surgically implanted medical device is essential, an unexpected reduction in cell capacity means that the device must be surgically removed and replaced much earlier than usual.
It should be understood that not all cells with imbedded current collectors will undergo a dramatic impedance increase that requires replacement of the cell. However, even without the problem of early cell replacement, the variability of cell impedance complicates the use of the cell. Powering critical implantable medical devices such as pacemakers, neurostimulators and drug infusing pumps, requires compactness and efficiency of circuit design. If the cells used are too variable in their output, the device would need to be larger and less efficient since the circuits must either increase in complexity to compensate for the output variability or must include capacitors which can provide additional energy storage.
Of course, it is known to provide cathodes without imbedded current collectors. For example, U.S. Pat. No. 3,440,110 issued to Arbter discloses a cathode assembly which includes a support ring into which a cathode material is pressed. This cathode assembly is then pressed into intimate contact with the bottom of the cell housing so that the housing itself can contact the cathode material and act as a current collector. However, in a high reliability electrochemical cell for use in critical medical device applications, reliance on this contact between the cathode and the case can still raise a concern about undesirable impedance variations.
It is, therefore, an object of the present invention to provide an electrochemical cell in which a cathode material which is subject to swelling during discharge has a current collector which will obviate cell impedance variability.